1. Field of the Invention
The invention relates generally to the field of atomic frequency standards and particularly to atomic frequency standards which are optically excited using a technology known as Coherent Population Trapping (CPT) Atomic Frequency Standards.
2. Description of Related Art
A CPT atomic frequency standard is an “atomic clock” based on the phenomenon of coherent population trapping (CPT). Like most clocks, atomic clocks use phenomena with a regular time period to measure time. In atomic clocks, the phenomena with the regular period involve atoms that make transitions between two energy levels at angular frequency ωo. In most atomic clocks realized up to now using alkali metal atoms, these energy levels are part of the ground state of the atoms. The angular frequency ωo of these transitions is typically in the microwave range, 6.834 . . . GHz for rubidium 87, for example. The transitions can be detected by several means and among others through emission or absorption or energy at the resonance frequency, or when excited at that resonance frequency, by means of effects on a light beam interacting with the same atoms.
In coherent population trapping, the atoms are subjected to circularly-polarized optical radiation at two angular frequencies ω1 and ω2 connecting the two levels of the ground state to a third level called the excited state. When the frequency difference (ω1−ω2) of the optical radiation fields is not exactly equal to the ground state resonance frequency ωo, the atoms are not trapped in the ground state. They can absorb energy from the optical radiation fields and enter the excited state. The resonance phenomenon in the ground state at frequency ωo is thus observed directly as a reduction in the transmitted radiation. When the difference frequency (ω1−ω2) is exactly equal to the atomic resonance frequency ωo in the ground state, the atoms cannot absorb the electromagnetic radiation or be excited to the excited state. As a consequence, there is a sharp decrease in the absorption of the transmitted light. This “bright line” in transmission is used to lock an radio-frequency oscillator to the difference frequency (ω1−ω2).
FIG. 1 is a block diagram of a CPT frequency standard 101 of the type disclosed in U.S. Pat. No. 6,320,427, cited in the Cross references to related applications. At the highest level, frequency standard 101 works as follows: The current source 125 driving laser 103 is modulated by microwave generator 127 at frequency ωo/2. This has the effect of creating, in the output spectrum of the laser, sidebands spaced symmetrically on each side of the laser carrier frequency. These sidebands are separated by ωo/2 and their amplitude is given by Bessel functions Jn. The two first sidebands called J1+ and J1− situated on each side of the carrier are thus separated by the frequency ωo. They are the sidebands used as the two circularly-polarized radiation fields ω1 and ω2. Under the excitation of these two sidebands, the atoms are trapped in the ground state, they cannot absorb the light from the laser, and virtually all of the light passes through resonance cell 111 to photodetector 113; when (ω1−ω2) is not equal to ωo, the atoms are not trapped in the ground state, much more of the light is absorbed by the atoms in resonance cell 111 and much less light reaches photodetector 113. Photodetector 113 produces a current which is proportional to the amount of light that falls on it, and the current from photodetector 113 thus indicates when (ω1−ω2) is equal to ωo or not.
Microwave generator 127 is modulated at a low frequency. The modulation causes the frequency separation (ω1−ω2) to vary periodically by a small amount and this in turn causes a low frequency periodic variation of the optical radiation at photodetector 113. This periodic variation is processed to lock the microwave generator to the atomic resonance at ωo. The frequency standard produced by clock 101 is derived from the locked frequency of the microwave generator.
Light originating from laser 103 which excites the atoms in resonance cell 111 must have certain properties in order to initiate the CPT process. The gas in cell 111 is excited by circularly-polarized light at the correct wavelength and optimum optical power. The correct wavelength is achieved by setting the temperature and drive current to the laser diode providing the light, the optical power of the laser beam is controlled by attenuator 107, and circular polarization is achieved by properly aligning quarter wave retarder 109 with regard to the plane of polarization of laser light 105. In the past one adjusted the optical power by using an attenuating material (film, glass, or otherwise) placed in the beam path to reduce its intensity. The attenuating material can be placed on either side of the quarter wave retarder. In a very small system, optimization of the optical intensity is adjusted by selecting a discrete optical attenuator. Best results are generally achieved using glass neutral-density filters, but these can be quite expensive and take up larger amounts of space. They also do not come in a very wide selection of values, so they must either be paired together, taking up even more space, or a sacrifice in optimum optical power must be made.
As described above, adjusting the optical intensity has been done in the past by installing and removing attenuators. Adjusting the circular polarization has been done by rotating the quarter wave retarder relative to the plane of polarization of laser light 105 and using an external linear polarizer or other appropriate means to determine the state of polarization resulting from the rotation. However, any calibration which requires that components of the device be replaced or that calibration components be added to the device and manipulated in the device is undesirable. For example, extra space is required for the combinations of attenuators that are needed to attain the optimum optical power and for the equipment required to analyze the polarization of the light entering resonance cell 111. Further, installation and removal of the analysis equipment and/or installation and removal of the attenuators often disturbs the alignment of CPT frequency standard 101 generally and of quarter-wave retarder 109 in particular. Another related problem is that adjustment techniques which require installation and/or removal of attenuators or analysis equipment cannot be performed automatically by the CPT frequency standard itself. What is needed, and what is provided by the present invention, is a technique for adjusting the optical intensity and circular polarization of the laser beam which requires neither installation and removal of the analysis equipment nor use of attenuator 107. As will be apparent from the foregoing discussion, such a technique is useful not only in CPT frequency standards, but in any application in which circularly-polarized light of precisely-controlled intensity is required. It is thus an object of the invention to provide such a technique.
An important property of optical radiation is the polarization state. There are two basic polarization states: linear polarization and elliptical polarization. As noted above, in the present context, we are chiefly interested in circular polarization, a special case of elliptical polarization. In circular polarization, the electrical field rotates around the line upon which the optical wave propagates, unlike linear polarization in which the electrical field of the optical wave moves in planes that contain the line along which the optical wave propagates. A good elementary discussion of polarization was found in August, 2004 at www.meadowlark.com/AppNotes/Appnote%20PDF/Basic%20Polarization%20Techniques%20and%20Devices.pdf. That discussion is hereby incorporated by reference in the present patent application.
The linear polarizer has a polarizing axis, and when light propagates through a linear polarizer, the emergent light is linearly polarized in the plane of the polarizing axis. If light that is already linearly polarized is input to a linear polarizer, only the component of the linearly-polarized light that is parallel to the polarizing axis emerges; the remainder of the light is absorbed or reflected. Thus a linear polarizer can thus be used to attenuate linearly-polarized light.
Circularly-polarized light is produced by passing light through a circular polarizer. A circular polarizer has two components, a linear polarizer and a quarter-wave retarder, which are assembled in a specific orientation. A quarter-wave retarder is made from a birefringent, uniaxial material having two different refraction indices. Light polarized along the direction with the smaller index travels faster and thus this axis is termed the fast axis. The other axis is the slow axis. In the circular polarizer, there is a fixed orientation of the axis of polarization of the linear polarizer to the fast axis of the quarter-wave retarder. An orientation of 45° results in the most efficient conversion of the linearly-polarized light emerging from the linear polarizer to circularly-polarized light, but circular polarization can occur at other orientations as well.